Left 4 Dead 2 is on the supported games list, but when I start it, it just runs on the monitor, the rift doesn’t react. Vorpx seems to generally work, no errors, Fallout 4 for example works.
Edit: now it starts, but when I want to start a game, it says “Steam must be running”. It doesn’t say that when I am playing without rift.
I am having the same issue in your edit. I’ve tried running the exe directly and using a vorpx desktop shortcut. I can click past valve guy and the opening cinematic but it stops on the loadnig screen. Taking the headset off shows L4D2 now as just a windows dialog box “Steam has to be running to play this game(paraphrase)”